A polybutylene terephthalate (PBT) resin has excellent mechanical and electrical properties, heat resistance, weather resistance, water resistance, and resistance to chemicals and solvents. Therefore, the resin is widely used as an engineering plastic in various purposes such as automotive parts and electric or electronic device parts. In recent years, the use of the PBT resin as an exterior package or a container material (a housing material) for protecting a substrate of an electronic device part or others has increased. When it is necessary to seal the package or the container in such a case, bonding or welding methods are usually conducted. Among the welding methods, a laser welding method enables bonding (or joining) by melting only an area to be welded without vibrating or heating all over a product, differently from a vibration welding or an ultrasonic welding. Therefore, in recent years, the laser welding method has been attracting much attention as a method suitable for bonding of an exterior package (or a container) for protecting a precision electronic device part, or others.
However, since the polybutylene terephthalate resin is a material having a relatively low laser transmissivity, a molded product formed from the resin is limited in thickness for applying a laser welding to the molded product. In order to respond to requests, for example, to allow a laser welding of a thick product, the molded product of the polybutylene terephthalate resin has required an improved laser transmissivity.
For example, Japanese Patent Application Laid-Open No. 26656/2001 (JP-2001-26656A, Claims and paragraph numbers [0003] and [0020]) discloses a process for producing a united molded product, which comprises uniting (A) a molded product formed from (a) at least one polyester-series copolymer with (B) other molded product by a welding processing. In the process, the polyester-series copolymer (a) is selected from the group consisting of a polybutylene terephthalate-series copolymer having a melting point within a specific range, a polyethylene terephthalate-series copolymer having a melting point within a specific range, and a polyethylene naphthalate-series copolymer having a melting point within a specific range. This document mentions that a PBT-series copolymer having a melting point within the range of 170° C. to 220° C. has a low heat of fusion and can melt by using a lower fusion energy, particularly due to a high light transmittance in a laser welding method, and that the bonding using a lower fusion energy improves the welding strength and increases the range of welding conditions. In addition, this document describes that a homopolybutylene terephthalate resin has a small laser-welding strength. However, in the case of the copolymer, although the transmissivity is improved to some degree, there is a possibility of deterioration in moldability.
Moreover, Japanese Patent Application Laid-Open No. 292752/2003 (JP-2003-292752A, Claims and paragraph number [0029]) discloses a laser-weldable resin composition which comprises (A) a polybutylene terephthalate or a polybutylene terephthalate-series resin comprising a polybutylene terephthalate and a polybutylene terephthalate copolymer and (B) at least one resin selected from the group consisting of a polycarbonate resin, an acrylonitrile-styrene copolymer, a polyphenylene oxide, a styrene resin, an acryl resin, a polyether sulfone, a polyarylate, and a polyethylene terephthalate resin. In the composition, the proportion of the resin (B) is 1 to 50% by weight relative to the total amount of the resins (A) and (B). Moreover, this document mentions that the resin composition may contain, as a mold-release agent, a montan wax, a metal soap (such as lithium stearate or aluminum stearate), a higher fatty acid amide (such as ethylenebisstearylamide), a polycondensate of ethylenediamine, stearic acid, and sebacic acid, and others. In the resin composition described in this document, when the mixing ratio of the resin (B) is small, the effects of the resin (B) are unclear. A large mixing ratio of the resin (B) sometimes ensures improvement in the laser transmissivity of the composition, while there is a possibility that the properties as the PBT are lost (e.g., the chemical resistance or the moldability is deteriorated).
Incidentally, Japanese Patent Application Laid-Open No. 290476/2000 (JP-2000-290476A, Claims and paragraph number [0022]) discloses a weldable resin composition which comprises 100 parts by weight of a thermoplastic polyester (such as a polybutylene terephthalate) and 0.02 to 50 parts by weight of at least one compound selected from the group consisting of a hydrocarbon-series compound, an alkylenebis (fatty acid amide)-series compound, a fatty acid ester-series compound (e.g., a partly saponified ester of montanic acid) a fatty acid and a metal salt thereof, a glycol-series compound, a polyester elastomer-series compound, a polyamide elastomer-series compound, a polyolefinic compound, and an epoxy-series compound. Although this document mentions that the resin composition or a molded product thereof can be utilized for a welding such as an ultrasonic welding, a vibration welding, a spin welding, a high-frequency welding, an induction welding, a hot plate welding, a hot-air welding, a friction welding, or an impulse welding, an application of the resin composition to a laser welding is not supposed.
Moreover, Japanese Patent Application Laid-Open No. 184410/1994 (JP-6-184410A, Claims) discloses a polybutylene terephthalate resin composition comprising (A) a polybutylene terephthalate resin having a terminal carboxyl group of not more than 15 equivalent/106 g and (B) 0.01 to 3% by weight of a fatty acid ester-series compound (e.g., an ester of a fatty acid having a carbon number of 12 to 32 with a mono- or polyvalent alcohol) for imparting improved feeding in screw and mold release property to the resin (A). Further, Japanese Patent Application Laid-Open No. 170933/1993 (JP-5-170933A, Claims) discloses that an organic additive that can show internal lubrication and/or mold release effects (e.g., a metal salt of a fatty acid) may be added to a thin polyester molded product molded from a specific polybutylene terephthalate copolymer. In these documents, both the fatty acid ester-series compound and the metal salt of the fatty acid are used as a lubricant. These documents are silent on the laser welding of the resin composition or the thin molded product.